The origins of our reproductive lives are a mystery. Why are a quarter of a billion sperm cells needed to fertilize one egg? Are women really fertile for only a few days each month? How long should babies breastfeed? In his new book “How We Do It,” Field Museum of Natural History curator Robert Martin draws on 40 years of research in biological anthropology to locate the roots of everything from our sex cells to the way we care for newborns. He examines the procreative history of humans, as well as that of our primate kin, to reveal what’s really natural when it comes to making and raising babies. Martin is joined in conversation by Re:sound host Gwen Macsai.